Forming fabrics for the production of quality paper originally comprised a woven metal mesh, such as phosphor bronze, but for a number of years the metal mesh has been replaced by synthetic textile materials, and forming fabrics woven from synthetic yarns are in widespread use. The ever increasing demand for better quality paper has traditionally resulted in a reduction of the diameters of the yarns with a corresponding increase in the number of yarns. While simply increasing the number of picks will improve fiber support, a point is reached where drainage is reduced beyond an acceptable level. As a result, smaller diameter yarns must be used to maintain good drainage while increasing the number of fiber support points. While this has resulted in improved paper quality, the stretch resistance of the fabrics has gone down exponentially with a reduction of yarn diameter, and if the mesh becomes too fine, the fabric will be weakened and its wear resistance materially reduced.
In addition to the foregoing, a fabric used in the forming area of a papermaking machine must be dimensionally stable in both the machine and cross-machine directions of the fabric. Instability in the machine direction is reflected as stretch, whereas instability in the cross-machine direction may be seen as fabric width contraction at the high tension side of the couch roll or drive roll. Instability in either direction resulting in a dimensional change greater than 1% will generally result in the early failure of the fabric. In fact, some papermaking machines have a machine direction stretch tolerance as low as 0.25%.
Cross-machine instability is a result of an interchange of crimp from the machine direction yarns into the cross-machine direction yarns brought about by tension developed in the machine direction yarns when the fabric is in use. As the machine direction yarns become straighter, the fabric loop becomes longer, and as the crimp in the cross-machine direction yarns is increased, the fabric contracts and becomes narrower. Fabric contraction of any amount is undesirable and if in excess of 0.3% is generally undesirable. Typically, an endless fabric is woven with some degree of crimp in the machine direction. The fabric is then heat set in the finishing process while being overstretched in the machine direction. This overstretching and heat setting is to remove as much crimp as practical from the machine direction yarns and thereby maximize stability, i.e., minimize stretch and contraction on the papermaking machine. At best, such fabrics are a compromise both with respect to dimentional stability and wear characteristics.
The approach taken by the industry towards solving the problems encountered with synthetic forming fabrics has been to go to the use of duplex or double layer fabrics. However, in a duplex fabric, such as that taught in U.S. Pat. No. 3,915,202, Oct. 28, 1975, any yarns undulating through both surfaces of the fabric must serve both as a sheet forming yarn (preferably fine), as well as a wear yarn (preferably coarse). This results in a compromise with respect to both sheet quality and wear, and despite efforts to enhance fabric stability by varing the modulus of elasticity of the yarns, the majority of crimp remains in the machine direction yarns.
It has also been proposed in Japanese Patent No. 40 15842, dated July 22, 1965, to join a conventional 2/1 twill fabric with a plain weave substrate, the two fabrics being joined either by bonding them together utilizing a bonding agent, or by utilizing selected yarns of the 2/1 twill sheet forming fabric for stitching. Bonded fabrics are not practical, and where stitching is employed, the fine yarns dropped out of the sheet forming fabric for stitching purposes results in holes in the fiber support system. In addition, if the crimp in the machine direction yarns of the sheet forming surface is retained, the fabric stretches; and if the crimp is reduced, inadequate sheet support and wire marking is encountered.
In contrast to the foregoing, the present invention relates to an improved double layer fabric having two functional sides, the pulp receiving or sheet forming side consisting of bi-crimped yarns selected to be conducive to improve sheet characteristics, and a machine or wear side consisting of coarser yarns woven and interlaced with the sheet forming side in a manner to enhance stability, particularly in the machine direction, and also the wear characteristics of the fabric.